## **Core Concept**
Diphtheria is an infectious disease caused by *Corynebacterium diphtheriae*, a Gram-positive bacterium. The primary mode of transmission is through respiratory droplets from infected individuals. The bacteria can colonize the respiratory tract, producing a toxin that causes the characteristic symptoms.

## **Why the Correct Answer is Right**
The correct answer, **C. Cases**, indicates that the most common source of diphtheria is individuals who are already infected and symptomatic. *Corynebacterium diphtheriae* is highly contagious, and infected individuals serve as the primary reservoir for the bacteria. Asymptomatic carriers can also spread the disease, but cases, especially those with respiratory symptoms, are the most significant source of infection.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, although carriers can be a source of infection, they are not the most common source compared to symptomatic cases.
- **Option B:** This option is incorrect as there is no commonly recognized source of diphacteria labeled simply as "" in standard medical literature.
- **Option D:** This option is incorrect because animals are not typically considered a common source of diphtheria infection in humans. *Corynebacterium diphtheriae* is primarily spread from human to human.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that diphtheria is preventable through vaccination, specifically the diphtheria toxoid vaccine, which is often given in combination with tetanus and pertussis vaccines (DTaP). Asymptomatic carriers of *Corynebacterium diphtheriae* can be identified through throat swabs and are treated with antibiotics to prevent transmission.

## **Correct Answer:** . Cases